1. Field of the Invention
The present invention relates to a non-aqueous secondary battery comprising a negative electrode made of an active negative electrode material capable of intercalating/deintercalating lithium ion, a positive electrode made of spinnel type lithium manganese oxide as a main active positive electrode material and an electrolyte containing a non-aqueous solvent.
2. Description of the Related Art
In recent years, as a battery for portable electronic and communications equipment such as portable telephone and note type personal computer there has been practically used a rechargeable non-aqueous battery having a light weight and a high capacity such as lithium ion battery comprising an alloy or carbon material capable of intercalating/deintercalating lithium ion as an active negative electrode material and a lithium-containing transition metal oxide such as lithium cobalt oxide (LiCoO2), lithium nickelate (LiNiO2) and lithium manganese oxide (LiMn2O4) as a positive electrode material.
Among the foregoing lithium-containing transition metal oxides as positive electrode material constituting the non-aqueous battery, lithium nickel oxide (LiNiO2) has a high capacity but is greatly disadvantageous in that it is inferior to lithium cobalt oxide (LiCoO2) in safety and properties. For example, lithium nickel oxide (LiNiO2) exhibits a high overvoltage. Further, lithium manganese oxide (LiMn2O4) occurs in abundance and thus can be available at a low price but is greatly disadvantageous in that it has a low energy density and manganese itself is dissolved at high temperatures. Thus, lithium manganese oxide (LiMn2O4) is inferior to lithium cobalt oxide (LiCoO2). Therefore, it is a main practice at present to use lithium cobalt oxide (LiCoO2) as a lithium-containing transition metal oxide.
However, as such a type of non-aqueous battery has been used not only for consumers"" small-sized apparatus such as small-sized video camera, portable telephone, note type personal computer and other portable electronic and communications equipment but also for large-sized apparatus such as hybrid automobile, lithium manganese oxide (LiMn2O4), which occurs in abundance and thus can be available at a low price, has been noted as a substitute for lithium cobalt oxide (LiCoO2), which occurs less than lithium manganese oxide. Under these circumstances, in order to eliminate the problem of low energy density lithium manganese oxide (LiMn2O4), JP-A-9-293538 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses the incorporation of lithium cobalt oxide (LiCoO2) or lithium nickel oxide in lithium manganese oxide (LiMn2O4) in an attempt to eliminate such a problem.
However, even the approach proposed in the above cited JP-A-9-293538 leaves something to be desired. Particularly important properties to be improved in a positive electrode comprising lithium manganese oxide (LiMn2O4) as an active positive electrode material are high temperature cycle properties and storage properties. Referring to high temperature cycle properties, many studies have been made of stabilization of crystal structure by the addition of foreign elements or other approaches. However, chromium or other effective substituent elements are harmful substances. Further, when these substituent elements are added in a large amount, it causes problems such as deterioration of energy density. Thus, no practical solutions have been found yet.
Referring to storage properties, lithium manganese oxide (LiMn2O4) reacts readily with the electrolytic solution to undergo self-discharging, eventually causing the production of gas that deteriorates the battery properties. This phenomenon occurs remarkably when the battery is stored discharged. Further, when stored at high temperatures, manganese is dissolved to produce a large amount of gas. No effective means for improving storage properties have been found yet.
The present invention has been worked out to solve the foregoing problems. An object of the present invention is to provide a non-aqueous secondary battery which undergoes inhibited self-discharging and exhibits excellent discharged storage properties and high temperature storage properties, a high discharge operating voltage, a high energy density and an improved safety despite the use of lithium manganese oxide (LiMn2O4) as a main active positive electrode material.
To this end, the non-aqueous secondary battery of the present invention comprises a positive electrode made of spinnel type lithium manganese oxide having crystal lattices partially substituted by magnesium or aluminum mixed with lithium cobalt oxide and a non-aqueous solvent having vinylene carbonate (VC) incorporated therein.
The spinnel type lithium manganese oxide acts as a strong oxidizing agent and thus reacts with the electrolytic solution or electrolyte salt to produce a large amount of gas. This not only deteriorates the properties of the battery but also produces abnormal inner pressure that deforms the battery and causes the leakage of electrolytic solution, deteriorating the safety of the battery.
However, the partial substitution of crystal lattices by magnesium or aluminum makes it possible to depress the activity of spinnel type lithium manganese oxide and hence minimize the deterioration of the battery during high temperature charged storage and the production of gas. The incorporation of lithium cobalt oxide, which acts to inhibit the reaction with the electrolytic solution, causes the reduction of the produced amount of gas and the voltage drop and hence the rise in the percent retention of capacity and percent recovery of capacity. As the added amount of lithium cobalt oxide increases, this phenomenon becomes more remarkable. The incorporation of vinylene carbonate (VC) in the non-aqueous solvent makes it possible to further lower the produced amount of gas because vinylene carbonate forms a decomposition product film mainly on the negative electrode to relax the reaction with the non-aqueous electrolyte. As a result, a non-aqueous secondary battery having excellent discharged storage properties and high temperature storage properties, a high discharged operating voltage, a high energy density and an enhanced safety can be obtained.
As the added amount of lithium cobalt oxide increases, the resulting buffering action becomes more remarkable. Thus, the added amount of lithium cobalt oxide is preferably 0.05 parts (5% by weight) based on the total weight of the active positive electrode material. It has generally be thought that since the discharged operating voltage of lithium cobalt oxide is lower than that of lithium manganese oxide, the discharged operating voltage of lithium manganese oxide mixed with lithium cobalt oxide is lower than that of lithium manganese oxide alone. However, since lithium cobalt oxide has a better electronic conductivity than lithium manganese oxide, the incorporation of lithium cobalt oxide causes a rise in the discharged operating voltage.
However, when lithium cobalt oxide is added in an amount of greater than 0.3 parts (30% by weight) based on the total weight of the active positive electrode material, the effect of lithium cobalt oxide itself becomes more remarkable, deteriorating the overcharging properties. Thus, the added amount of lithium cobalt oxide preferably falls below 0.3 parts (30% by weight). It is preferred after all that lithium cobalt oxide be added so as to satisfy the relationship 0.05xe2x89xa6B/(A+B) less than 0.3, preferably 0.05xe2x89xa6B/(A+B) less than 0.2 in which A represents the mass of spinnel type lithium manganese oxide having crystal lattices partially substituted by magnesium or aluminum and B represents the mass of lithium cobalt oxide.
As vinylene carbonate (VC) is added more, the thickness of the decomposition product film formed on the negative electrode increases, relaxing more the reaction with the non-electrolyte and hence making it possible to further reduce the produced amount of gas. However, when vinylene carbonate is added in too great an amount, the amount of resistive components on the surface of the electrode plates increases. Thus, it is necessary that the added amount of vinylene carbonate (VC) be restricted so as to reach a proper film thickness. The added amount of vinylene carbonate is preferably 0.03 parts (3% by weight) or less based on the total weight of the non-aqueous solvent.
The retention of capacity per the atomic ratio of lithium and magnesium or lithium and aluminum to manganese in the spinnel type lithium manganese oxide substituted by magnesium or aluminum ((Li+Mg)/Mn or (Li+Al)/Mn) at a high temperature (60xc2x0 C.) was experimentally determined. As a result, as the atomic ratio increases, the retention of capacity at a high temperature increases. However, when the atomic ratio increases beyond 0.62, the retention of capacity at a high temperature no longer increases. On the other hand, as the atomic ratio increases, the ratio of capacity to active positive electrode material decreases. Thus, the upper limit of the atomic ratio ((Li+Mg)/Mn or (Li+Al)/Mn) is preferably 0.62 or less.
Further, as the atomic ratio ((Li+Mg)/Mn or (Li+Al)/Mn) decreases, manganese is dissolved more at high temperatures and deposited on the negative electrode, deteriorating the surface conditions of the electrode plate. Thus, the incorporation of vinylene carbonate in the electrolytic solution causes vinylene carbonate to be electrolytically decomposed on the negative electrode side to form a film on the negative electrode, inhibiting the dissolution of manganese. However, in the area where manganese is dissolved more, the effect of dissolution of manganese appears more than the amount of the film formed on the negative electrode by vinylene carbonate, causing a sudden drop of retention of capacity. This phenomenon occurs in the area where the atomic ratio ((Li+Mg)/Mn or (Li+Al)/Mn) is in the vicinity of 0.54. It is thought that as the added amount of vinylene carbonate increases, the atomic ratio is shifted toward smaller side. However, since the increase in the added amount of vinylene carbonate has an adverse effect, the lower limit of the atomic ratio ((Li+Mg)/Mn or (Li+Al)/Mn) is preferably 0.54 or more.
As above described, the upper limit of the atomic ratio is determined by at a high temperature and the ratio of capacity to active positive electrode material. And the lower limit of the atomic ratio is determined by the relationship between vinylene carbonate and the retention of capacity.
After all, from the foregoing standpoint of view, the atomic ratio is determined such that the atomic ratio satisfies the relationship 0.54xe2x89xa6(Li+M (M=Mg, Al))/Mnxe2x89xa60.62.
The positive electrode of the invention made of spinnel type lithium manganese oxide substituted by magnesium or aluminum mixed with lithium cobalt oxide is greatly characterized by that it can be applied not only to a non-aqueous secondary battery comprising an organic electrolytic solution but also to a non-aqueous battery comprising a polymer solid electrolyte. A polymer solid electrolyte has a greater viscosity than an electrolytic solution and thus can be hardly retained by the positive electrode singly made of spinnel type lithium manganese oxide substituted by magnesium or aluminum. However, the positive electrode made of spinnel type lithium manganese oxide substituted by magnesium or aluminum mixed with lithium cobalt oxide can be formed thinner and thus can eliminate the problem of poor retention of electrolyte.
As the polymer solid electrolyte there is preferably used a solid electrolyte obtained by gelatinizing a polymer selected from the group consisting of polycarbonate-based solid polymer, polyacrylonitrile-based solid polymer, copolymer or crosslinked polymer comprising two or more of these polymers and fluorine-based solid polymer such as polyvinylidene fluoride (PVdF), a lithium salt and an electrolytic solution in combination.